1. Field of the Invention
The present invention generally relates to a method for adjusting the depth of scanning field to an optimally increased range. Especially, the invention is a scanner assembling method which increases and adjusts the depth of scanning field to an optimal range, Thus, the scanner can obtain good quality of scanning images even if the original document is wrinkled.
2. Background Description
Nowadays, the scanning mechanism and technology have improved dramatically. The scanning device is generally integrated to our daily life. Users can capture the images from documents and transfer the images to electrical files. Thus, the electrical files can be sent to others. Usually, the scanning device will employ some optical elements and scanning technologies. The employed optical technologies and elements are needed to be adjusted for scanning documents to acquire scanning images via a precise scanning mechanism. The scanning device is able to record the scanning image by an image generating means. In view of the above, the initial adjustment and assembly of the optical technologies and elements, are very important to the whole scanning processes.
Referring to FIG. 2, the traditional adjustment and assembly of a scanning device are illustrated as follows: FIG. 2 shows the traditional adjustment and assembly of the scanning mechanism along with the chart of MTF (Modulation Transfer Function) and DF (Depth of Field) curve. The scanning mechanism 22 includes a glass plate 221, lens 222 and an image generating means 220. The correction document 24 is placed on the glass plate 221.
Further, FIG. 2 also shows the chart of MTF and the depth of field of scanning position. The value of MTF represents resolution of the scanning module of a scanning device. MTF is generated from a mathematical formula. Higher value of MTF means higher resolution. Lower MTF can be deemed as a lower resolution of the scanning module, and vice versa. Generally, the shape 200 of the MTF-DF curve varies from different characteristics of the scanning module 22. Different positions of lens 222 also change the shape 200 of the MTF-DF curve. Therefore in the actual practice, the value of MTF should be kept above a certain value when the scanning module 22 is assembled. The certain value is shown in FIG. 2, as legend 201. This value is generated by the acceptance of resolution by human eyes. If the value of MTF is maintained above legend 201, the value of DF will range between the legend 204 and legend 205. Thus, the range between legend 204 and legend 205 is defined the tolerance of DF, and be presented by legend 202.
As stated in the above, when the scanning device is under adjusting process, a correction document 24 is placed on the glass plate 221. There are drawings and lines in the correction document for adjusting the scanning image. The scanning device also includes a light source (not shown in the figure). The light source projects light to the glass plate 221 and the light is reflected by the correction document 24. Thus, the reflected light, passes through an optical path 223 and lens 222 and finally been generated to a scanning image in a scanning generating means 222. The scanning generating means 222 is a CCD (charged coupled device). By an output device (not shown in the figures), the correction technician will: know the value of the MTF. This value of MTF represents the resolution of the correction document 24 which is placed on the glass plate 221. If the correction person adjusts the position of lens 222, the shape 200 of MTF-DF will move upward or downward, following the changing position of lens 222. In this way, the related MTF of the scanning module 22 will be achieved. Therefore, the correction person can adjust and fix the lens 222 to achieve the maximum value of MTF of the scanning module. The fixed position of the lens 222 is the best position to achieve the optimal resolution. This fixed position is also the optimal depth of field which is the legend 203.
Recently, users ask higher quality and more functions to the scanning device. For example, scanning devices with increased optical resolution and ability to scan different scanning objects are preferred by the buyers. However, if the manufacturer would increase the optical resolution, the resolution of the image generating means (e.g., CCD) and lenses should be improved together. If the resolution of the lens is increased, the optical depth of field will be decreased and range of the depth of field 202 will be smaller. Therefore, if the scanning document is wrinkled, the wrinkled document will easily be out of the range of the depth of field 202. Thus, the scanning image will be fizzy in this situation as illustrated in FIG. 1A. As shown in FIG. 1A, a scanning object 12 is placed on a glass plate 10. Because the scanning object 12 is not smooth, the portion 14 will easily be out of the range of the tolerant depth of field of the scanning device. Thus, the portion 14 is fuzzy. Moreover, if the scanning device is employed to scan books or magazines the same fuzzy effects will happen in the binder portion. As shown in FIG. 1B, when the book 16 is opened above the glass plate 10, the binder 18 of the book 16 will be above the glass plate at a certain height. The binder will possibly be out of the depth of field and cause the scanning image to be fuzzy and unreadable.
It is therefore an object of the present invention to provide a method of correction and assembly of a scanning device.
According to the present invention after a correction adjustment of the scanning module, a glass plate of a scanning module is placed at a lowest tolerant point of MTF-DF curve in order to improve the range of the optical depth of field of the scanning device. Therefore, users can employ the maximum tolerant range of DF to scan a wrinkled document or the book as shown in FIGS. 1A and 1B.